THE ENDOCRINE SYSTEM 213 



adult it is reduced to fatty and fibrous strands. Histologically 

 the thymus is quite different from the thyroid and parathyroid 

 glands and shows a general resemblance to a lymph gland. It is 

 distinguished by the presence in the medullary region of peculiar 

 structures known as thymic or Hassall's corpuscles, consisting of 

 rounded groups of degenerated cells bounded by flattened 

 epithelial cells, whose function is unknown. The development 

 and fate of the thymus would seem to imply some relation with 

 the gonads. However, while removal of the gonads results in a 

 hypertrophy of the thymus and thus seems to remove an inhibi- 

 tion to its continued growth, removal of the thymus in young 

 guinea pigs has no effect on general body growth and fails to 

 cause hypertrophy of the gonads. Feeding thymus to frog 

 tadpoles stimulates growth and causes them to grow to unusual 

 size before metamorphosis. Pigeons with defective thymus 

 glands lay eggs deficient in shell, a condition that can be corrected 

 by feeding dried thymus. Much uncertainty exists regarding 

 the exact role of the thymus in the endocrine complex. The 

 most recent work indicates that administration of an extract of 

 thymus increases fertility in adult rats and that when given 

 continuously through successive generations it produces marked 

 precocity in growth and development. 



Adrenal Glands. — The adrenal glands of mammals are paired 

 and located near, or in contact with, the kidneys, with which, 

 however, they seem to have no functional relation. In man, each 

 adrenal is a triangular-shaped organ capping the anterior end 

 of the kidney (Fig. 135). In the rat they are spherical in shape 

 and are attached to the dorsal body wall just in front of the 

 kidneys. Two general regions are recognized: a central, medul- 

 lary region, and an outer, cortical region. These two regions 

 differ in their embryological origins and also in their functions. 

 In the frog (Fig. 134) the adrenal gland consists of a long narrow, 

 orange-colored band embedded in the ventral face of each 

 kidney. The cortical and medullary cells can be recognized, but 

 they are intermingled. 



The cells of the medullary region of the adrenal gland are 

 stained by salts of chromium and are therefore referred to as 

 chromaffin or chromophile cells. The staining reaction is due to 

 the presence of a substance known as epinephrine or adrenalin, 

 which has been prepared in a pure state and used in medicine 



